As the computer keyboard industry matures, there is an increasing drive among keyboard manufacturers to produce lower cost keyboards. Traditionally, manufacturers have produced a keyboard 10 such as that shown in FIG. 1. One such prior art computer keyboard is disclosed in U.S. Pat. No. 4,560,844 granted to Takamura on Dec. 24, 1985.
Keyboard 10 includes multiple keys 12 mounted in a housing 14, which includes a rigid metal backing plate 16, a rigid metal or plastic mounting plate 18, and a rigid plastic enclosure 20. Keyboard 10 also has a switch membrane 22 and a dome sheet 24 positioned between backing plate 16 and mounting plate 18.
Mounting plate 18 has multiple key supports 26 into which keys 12 are slidably mounted so that keys 12 can be moved from rest positions to activated positions.
Switch membrane 22 comprises multiple switch contacts positioned beneath respective keys 12. The switch contacts are actuated upon depression of these keys. Dome sheet 24 comprises multiple resilient domes 28 which project upward to bias keys 12 to their rest position. Domes 28 collapse when keys 12 are depressed and rebound to their original form when keys 12 are released by the user to provide the "spring-like" feel of the computer keys. When the keys are depressed, switch membrane 22 generates an electric signal from an actuated switch contact to an electrical circuit, such as microprocessor, which is also provided on keyboard 10, but not shown in this figure.
Backing plate 16 provides the support for mounting plate 18, switch membrane 22, and dome sheet 24.
The prior art keyboard of FIG. 1 has a drawback in that it is relatively costly to produce. Housing 14 requires three structural components: (1) backing plate 16, (2) mounting plate 18, and (3) enclosure 20. These three housing components contribute significantly to the overall keyboard cost. Additionally, assembly time is greater because employees or machines must handle and assemble multiple separate housing components during the construction of a single keyboard.
Recognizing this, keyboard manufacturers have attempted to reduce the number of components employed in their computer keyboards. For example, U.S. Pat. No. 4,876,415 granted to Clancy on Oct. 24, 1989 combines a portion of the keyboard enclosure with the backing plate to eliminate one structural layer of the housing. U.S. Pat. No. 4,760,217 granted to Suzuki on Jul. 26, 1988 proposes a similar structure in that a back portion of the enclosure also functions as the backing plate for the computer keyboard. However, both of these patents still require a mounting plate to support the keys in addition to the combination enclosure/backing plate.
Prior art keyboards have another disadvantage in that they are noisy to operate. When a computer key is depressed, the plastic key cap typically strikes the plastic or metal mounting plate. This plastic-to-plastic contact causes a "clicking" sound which is tolerable, but undesired.
A separate problem associated with conventional keyboards concerns switch contact designs. One prior art switch contact 30 is illustrated in FIGS. 2A and 2B. Switch contact 30 is formed in switch membrane 32, which includes an upper flexible layer 34, a spacer layer 36, and a lower, rigid or flexible layer 38. A first conductive pad 40 is provided on upper layer 34 and a second conductive pad 42 is provided on lower layer 38 to face first conductive pad 40. Spacer layer 36 separates conductive pads 40 and 42 to provide an insulative air gap therebetween. Conductive pads 40 and 42 thereby constitute switch contact 30.
When a transverse force F is applied to upper flexible layer 34 at switch contact 30 (FIG. 2B), first conductive pad 40 is brought into electrical contact with second conductive pad 42. Upper flexible layer 34 is deformed into a concave shape as shown and electrical contact between first and second conductive pads 40 and 42 is made at a single point 44.
The conventional switch contact of FIGS. 2A and 2B have an inherent drawback in that electrical contact is made initially only at a single point 44. Additional transverse force is often required to effectuate more surface contact between first and second conductive pads 40 and 42. This additional force is undesired because it contributes to user fatigue over a prolonged period of keyboard operation. A second drawback to this prior art switch contact is that foreign debris or particles may become lodged on conductive pads 40 and 42 at or near the point of contact 44, thereby preventing the desired electrical contact between these two conductive pads.
A second conventional switch contact design is illustrated in FIG. 3. A computer key 48 comprises a switch contact 50 constituted by conductive traces 52 and 54 which are spaced on a printed circuit board 58 in an adjacent, but electrically isolated, manner and conductive layer 56 mounted underneath collapsible dome 60. When computer key 48 is depressed, dome 60 buckles and conductive layer 56 is brought into an electrical contact with both conductive traces 52 and 54, thereby electrically shorting traces 52 and 54. U.S. Pat. No. 4,677,268, U.S. Pat. No. 4,760,217, and U.S. Pat. No. 4,814,561 all disclose computer keys similar to that shown in FIG. 3 in which conductive material is mounted to the dome to constitute part of the switch contact. The disadvantage of this switch contact construction is that dome sheets having conductive material fixed in individual domes are more expensive than dome sheets without the conductive material. Accordingly, a computer keyboard which incorporates the switch contact of FIG. 3 is relatively more expensive.
This invention provides a low cost keyboard having a minimum number of components. This invention also provides improved and low cost switch contacts for a computer key structure which substantially reduce problems associated with contamination.